A carbon nanotube discovered recently is a tubular material, which is ideally formed with a sheet structure of a hexagonal carbon lattice (a graphene sheet) parallel to an axis of the tube. Further, the carbon nanotube may be a multi-layered tube formed with plural sheets mentioned above. In theory, the carbon nanotube is expected to exhibit a metallic or semiconducting property depending upon the connection type of the hexagonal carbon lattice and the diameter of the tube, and is expected as a future functional material.
A material having a diameter of 1 μm or less, which is thinner than a carbon fiber, is commonly referred to as a carbon nanotube, and is thereby distinguished from the carbon fiber. However, there is no specifically clear border therebetween. In a restricted meaning, a tube formed with a graphene sheet of a hexagonal carbon lattice parallel to an axis of the tube is referred to as a carbon nanotube. (This restricted meaning is applied to a carbon nanotube in the present invention.)
In general, the carbon nanotube defined by the restricted meaning is further classified. A tube formed with a sheet of a hexagonal carbon lattice is referred to as a single-wall carbon nanotube (hereinafter, sometimes simply referred to as an “SWNT”), and a multi-layered tube formed with plural sheets of a hexagonal carbon lattice is referred to as a multiwall carbon nanotube (hereinafter, sometimes simply referred to as an “MWNT”). The method and conditions of synthesis determine, to some extent, the structure of the carbon nanotube to be obtained.
In particular, the SWNT has attractive diversity exhibiting a metallic or semiconducting property in accordance with a chiral vector, and thus has been principally considered to be applied to an electric and electronic element (see, for example, “Basics of Carbon Nanotube” authored by Yahachi Saito and Syunji Bando (1998), Corona Publishing Co., Ltd.). An attempt to improve the property of a field emitting element by utilizing the efficient field electron emission property is in the advanced stage (see, for example, K. Matsumoto et al., Extended Abstracts of the 2000 International Conference on Solid State Devices and Materials (2000), pp. 100–101). However, the SWNT has not been sufficiently studied so far with respect to optical applications.
In the case of application to an electric and electronic element, a minute probe can access a single carbon nanotube. On the other hand, in the case of optical application, access is principally made to bulk carbon nanotubes by using a luminous flux condensed to a diameter of several hundreds of nm to several tens of μm. The primary reasons for the delay in optical application of the SWNT as compared with electric and electronic element applications may be due to the difficulty in obtaining a high purity sample of the SWNT required for optical evaluation, and the difficulty in forming an optically uniform film as the SWNT is hard to dissolve in solvents. There is a report of evaluation of a nonlinear optical constant aimed at optical application of the SWNT. However, in this report, an SWNT in a solution state is evaluated at 1,064 nm, 532 nm and 820 nm, which are not in a resonant region, and there is no report of remarkable nonlinearity that promises practical use (X. Liu et al., Appl. Phys. Lett. 74 (1999), pp. 164–166; Z. Shi et al., Chem. Commun. (2000), pp. 461–462).
The SWNT is known to have absorption at a wavelength of 1.8 μm, which is in an optical communication wavelength region (1.2 to 2 μm) (H. Kataura et al., Synth. Met. 103 (1999), pp. 2555–2558). If the resonant effect in this absorption band can be directly utilized, remarkable nonlinearity can be realized in a band of these wavelengths.
On the basis of the above-described consideration, we have studied application of the SWNT to an optical element operating in an optical communication wavelength region.